describe('purchaseProduct', () => {
  const products = [
    { id: 1, name: 'Laptop', price: 1000, stock: 5 },
    { id: 2, name: 'Mouse', price: 25, stock: 0 },
    { id: 3, name: 'Keyboard', price: 75, stock: 10 },
  ]

  it('successfully purchases product', () => {
    const result = purchaseProduct(1, 1500, products)
    expect(result).toEqual({
      _tag: 'Right',
      right: { product: products[0], remainingBalance: 500 }
    })
  })

  it('returns error for non-existent product', () => {
    const result = purchaseProduct(99, 1000, products)
    expect(result).toEqual({ _tag: 'Left', left: 'Product not found' })
  })

  it('returns error for out of stock product', () => {
    const result = purchaseProduct(2, 1000, products)
    expect(result).toEqual({ _tag: 'Left', left: 'Out of stock' })
  })

  it('returns error for insufficient funds', () => {
    const result = purchaseProduct(1, 500, products)
    expect(result).toEqual({ _tag: 'Left', left: 'Insufficient funds' })
  })
})